WO2008151367A1

WO2008151367A1 - A system for production of hydrogen

Info

Publication number: WO2008151367A1
Application number: PCT/AU2008/000838
Authority: WO
Inventors: Geoffrey David Will; Neville Charles Stephenson
Original assignee: Alternative Energy International Ltd
Priority date: 2007-06-12
Filing date: 2008-06-12
Publication date: 2008-12-18

Abstract

A batch system for the production of hydrogen, the system including a fuel container adapted for use as a reactor, the fuel including an iron-based fuel source, a steam inlet to the fuel container to provide steam at an elevated temperature and pressure wherein the reactor is a fluidised bed reactor.

Description

A SYSTEM FOR PRODUCTION OF HYDROGEN Field of the Invention.

The present invention relates to systems for the production of hydrogen and particularly to system for the efficient reduction of steam to hydrogen and inorganic salts.

Background Art.

Steam-metal reaction systems for commercial hydrogen production have been investigated since the late 1950's.

The inventors of the present invention have also been active in this area previously.

A previous invention described in International Patent Application No.

PCT/AUOO/00446 provides an electrochemical cell ("Cell 1") that produces hydrogen, steam and heat by the reduction of water, at a high PH range using a solution of electro positive half cell reactions, each of which enhances the effectiveness of the other half-cell reactions .

A development of that process is described in PCT/AU2004/001080 which provides an add-on cell, "Cell 2", and further reduces the steam to hydrogen and metallic salts, thus increasing the overall hydrogen production. This Cell 2 increases the overall hydrogen production. Cell 2 is known as an enhancement stage because of the enhanced output of hydrogen from the original process involved in Cell 1.

The present application is directed towards a unique process used in enhancing the hydrogen output of the system. The basic reaction involves the reduction of steam on the surface of metallic iron. This reaction is well documented in the chemical literature and leads to the formation of magnetite Fe₃O₄, hydrated Fe₂O₃ ferrous oxide FeO and other hydrated iron oxides.

The objectives of this application are to describe the production of continuous streams of hydrogen that would be commercially viable and to control the thermodynamic parameters that are involved. hi addition, for the design to be compatible with national safety and use standards and legislative requirements and the resultant chemical products to be reconstituted for further use.

It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country.

Summary of the Invention.

The present invention is directed to a system for the production of hydrogen, which may at least partially overcome at least one of the abovementioned disadvantages or provide the consumer with a useful or commercial choice.

With the foregoing in view, the present invention in one form, resides broadly in a system for the production of hydrogen, the system including a fuel container adapted for use as a reactor, the fuel including an iron-based fuel source, a steam inlet to the fuel container to provide steam at an elevated temperature and pressure wherein the reactor is a fluidised bed reactor.

The fundamental reactions assumed in the system of the present invention are as follows:

Fe + 4/3.H₂O => 1/3 Fe₃O₄ + 4/3.H₂ Heat Released: 56.6kJ/mol (1013kJ/kg Fe) Fe + 2.H₂O₀₎ => 1/2 Fe₂O₃ + 2.H_{2 (g)} Heat Released: 58.3kJ/mol (685kJ/kg Fe) Fe + H₂O ₍i₎ => FeO + H_{2 (g)} Heat Required: 18.8kJ/mol (337kJ/kg Fe)

Note the heat of each reaction described about is at a reference temperature of 25 °C and are less exothermic (or less endothermic in the case of FeO) at higher temperatures. At 25°C the reactions are slow and the nature of the oxide products varies according to the physical conditions. The system preferably operates on the basis of thermal decomposition of water to form hydrogen with the energy for the decomposition of the water gained from the steam provided at elevated temperature and pressure rather than electrolytic decomposition.

Apart from the expected magnetite (Fe₃O₄) and hematite (Fe₂O₃) oxide forms, ferrous iron oxide an other forms such as iron hydroxides, for example

Fe(OH)₃. xH₂0 may also form. The nature of the metallic products formed is typically dependant upon conditions such as wetness, operating temperature, operating pressure, time and the influence of contaminants in the feed iron.

The reactor of the present invention will typically be a batch reactor, wherein a pre-filled fuel canister can be installed into the system to function as the reactor vessel until the iron-based fuel material contained in the pre-filled canister is sufficiently exhausted. The concept behind the integrated fuel canister/reactor is a timed (typically approximately 24 hour) batch hydrogen reaction, utilising a vessel that can be used for either in situ regeneration of the spent fuel or removal for reloading and/or external fuel regeneration. The batch process eliminates the issues associated with continuous fuel handling and spent fuel handling operations. Such operations, although possible, and indeed provided for according to the present invention, greatly add to the complexity and cost of the system. Of more concern in any continuous handling system, is the increased chance that hydrogen leaks will occur. Any such leaks from the reaction system are highly likely to ignite, due to the elevated reaction temperature (which exceeds the auto-ignition temperature of hydrogen) and the thermodynamic properties of hydrogen which cause the hydrogen to heat up when vented to a lower pressure.

Steam is typically passed through the canister over the time period at a nominal ratio of steam per kilogram of iron fuel. To provide the necessary conditions for steam to pass evenly through the particle bed, the reactor vessel will normally be fluidized, and the most preferred method is by mechanical vibration. In a single pass, roughly 25% of the steam will generally be converted to hydrogen, depending primarily on reaction temperature and extent of fuel conversion. Heat will also normally be liberated by the reactions, so that the product gas will be hotter than the reaction steam. This excess heat can be used to pre-heat the inlet steam or be retained in-system to maintain the temperature of the reactants.

The steam/hydrogen product gas is preferably partly cooled by exchange with the incoming reaction steam via a pipe-in-pipe heat exchange arrangement. The steam is condensed from the steam/hydrogen mixture in a set of cooling coils. Heat is transferred from coiled tubes initially to steam vapour and then to deionised water which will preferably be provided in a lower part of each vessel, thereby generating the required steam and recycling enough heat to minimise the need for the addition of external heat. The condensate saturation temperature will be a function of the system pressure and thus is expected to increase up to approximately 150 - 180°C after the initial start-up period.

Condensed water vapour is separated from the hydrogen product in the condensate separator and recycled to the steam generator via the condensate holding tank and condensate recycle pump. The controls in the condensate recycle system influence how much condensate is recycled and to which part of the system generation circuit, thereby providing temperature control to the reactor vessel, which may otherwise tend towards thermal run-away.

The vapour condenser and steam generator are preferably integrated as a single (modular) unit operation. The design concept recognises the batch mode of the system of the present invention and continues encasing hydrogen piping inside steam piping/vessels to reduce the risk of a hydrogen leak to the atmosphere.

The vapour condenser/steam generated typically includes a plurality, preferably three, equally sized vessels which are pre-filled with deionised water. One or more coils are preferably installed in each vessel to facilitate heat transfer from the hydrogen/steam mixture and the water in the steam generator chamber. Typically, counter current heat exchange will occur between the hydrogen/steam mixture in the coiled tubes and the water/steam in the outer shell. In this manner, the final condensing temperature can be minimised while preserving the heat in the system. The use of a number of tall, relatively small diameter units, as opposed to a single vessel, preferably enhances the desired temperature stratification effect on the water side without the need to resort to complex baffle designs.

The product hydrogen gas passes on to the hydrogen holding tank for further use or treatment. The hydrogen is stored at the final system pressure, which will preferably not be greater than 1000 kPag. This product is deemed as "wet" because the dew point of the moisture will be not lower than the temperature of the tank, which will be influenced by the temperature of the gas coming from the condensate separator. In contrast, industrial hydrogen gas is typically expected to have a moisture dew point of somewhat less than -20°C. A pre-filled fuel canister may be provided. A continuous feed cartridge may typically be provided. The fuel will normally be iron particulate material. Other materials may be used for example a high surface area porous iron particulate.

The reactor is associated with a temperature regulation system to control the temperature in the reactor, normally by controlling the amount of heat provided from the product gas. In certain circumstances, the reactor may require cooling to prevent the reaction system "running away". Reconstitution of reaction products The chemical products produced at the catalyzed metallic surfaces are essentially oxides. These chemical products will generally be reconstituted in the present system in order to be at least partially recovered either for re-use in the system or for sale. These (generally) insoluble compounds can be reduced to metal by a number of well known methods such as the formation of soluble salts, for example, nitrates and subsequent recovery by electrolysis. Commonly used industrial methods presently use carbon (coke) or organics such as methane. However, these systems are undesirable due to the formation of oxides of carbon which are a contributor to global warming.

By provision of pre- filled fuel canister, the spent fuel canister can be removed from the system and a replacement canister installed. The spent fuel canister can then be connected to a reconstitution system for reconstitution of the fuel particles. Brief Description of the Drawings.

Various embodiments of the invention will be described with reference to the following drawings, in which:

Figure 1 is a schematic block diagram of a hydrogen steam-iron process according to a preferred aspect of the present invention, showing preferred operating conditions.

Detailed Description of the Preferred Embodiment. According to a particularly preferred embodiment, a system for the production of hydrogen is provided.

The reactor of the illustrated embodiment is a batch reactor, namely a pre-filled fuel canister installed into the system to function as the reactor vessel until the particulate iron fuel material contained in the pre-filled canister is sufficiently exhausted. The preferred embodiment has a canister containing approximately 150 kg of un-reacted iron particles which are oxidized over a 3-hour time frame, at a peak reaction temperature of approximately 550°C, to produce hydrogen. In the system of the illustrated embodiment, steam is passed through the canister over the nominal 3-hour period at a nominal ratio of 0.45kg of steam per kilogram of iron fuel, that is approximately 22.5 kg/hour. To provide the necessary conditions for steam to pass evenly through the particle bed, the reactor vessel is fluidized by mechanical vibration. In a single pass through the reactor of the illustrated embodiment, roughly 25% of the steam will be converted to hydrogen, depending primarily on reaction temperature and extent of fuel conversion. Heat is also liberated by the reactions, so that the product gas is hotter than the reaction steam.

The steam/hydrogen product gas is partly cooled by exchange with the incoming reaction steam via a pipe-in-pipe heat exchange arrangement. The steam is condensed from the steam/hydrogen mixture in a set of cooling coils. Heat is transferred from coiled tubes initially to steam vapour and then to deionised water in a lower part of each vessel, thereby generating the required steam and recycling enough heat to minimise the need for the addition of external heat. The condensate saturation temperature is a function of the system pressure and thus is expected to increase up to approximately 150 - 180°C after the initial start-up period.

Condensed water vapour is separated from the hydrogen product in the condensate separator and recycled to the steam generator via the condensate holding tank and condensate recycle pump. The controls in the condensate recycle system influence how much condensate is recycled and to which part of the system generation circuit, thereby providing temperature control to the reactor vessel, which may otherwise tend towards thermal run-away. The product hydrogen gas passes on to the hydrogen holding tank for further use or treatment. The hydrogen is stored at the final system pressure, which will preferably not be greater than 1000 kPag. This product is deemed as "wet" because the dew point of the moisture will be not lower than the temperature of the tank, which will be influenced by the temperature of the gas coming from the condensate separator. In contrast, industrial hydrogen gas is typically expected to have a moisture dew point was somewhat less than -20°C.

The two most important vessels in the system are the fuel canister/reactor and the vapour condenser/steam generator.

The fuel canister/reactor is a pressure vessel which is heated/cooled via a coil wrapped about the outside of the vessel. The coil will normally be secured in place using hemispherical retainers installed every approximately 300 mm along the length of the coil. This coil carries reaction steam from an inlet in the top flange and terminates near the bottom of the canister where the steam enters the vessel via a finely perforated (sintered) distributor plate. Additional heating is facilitated using electric radiant heating elements located in the chamber surrounding the fuel canister.

The inlet/outlet is configured as a pipe-in-pipe (double containment) arrangement and terminates at both the reactor and the condenser vessel inlet manifold and custom manufactured flanges, which provides separate channels and ports for the product gas and reaction steam to transfer to the appropriate locations. In both cases the reaction steam port completely encircles the steam/hydrogen outlet port, so any leak from this area bridges to the steam circuit rather than to the atmosphere. The steam circuit will operate at a higher pressure than the steam/hydrogen circuit, so any leaks will usually cause dilution of the product gas rather than hydrogen recycle into the reaction steam. If such a leak in severe enough it will eventually result in a reduction in steam supply to the reactor causing a loss of reactivity, either due to a loss of pressure differential and/or as a result of overloading the vapour condenser. Leakage events are to be determined from a loss of pressure differential between the steam/hydrogen circuit and the steam supply circuit.

Sealing of the mating flanges is achieved by a pair of single-use low carbon steel, metal O-rings seated into two concentric grooves. The inner ring seals the product gas outlet and the outer ring seals the reaction steam inlet. Low carbon steel does not have long-term compatibility with steam at high temperatures however it has been selected to ensure that the O-ring is softer than the flange surfaces. These O-rings are provided with a copper or soft chrome coating rather than the standard 8 micron zinc coating, because the zinc coating will fuse and rapidly oxidise once the vessel is heated above 450°C.

The canister is pre-filled with approximately 150 kg of particulate iron fuel. The reaction steam inlet is used to purge the system with dry nitrogen during start-up, then to provide heating and once the reaction is fully underway to provide cooling to avoid excessive operating temperatures.

Due to the low gas velocities in the fuel canister, it is expected that there will be very little solids carry over. Any particulate carryover will generally be filtered out of the product gas by a sintered plate in the top of the vessel. The plate is normally retained in a small lip in the top flange of the vessel and held down and sealed against the mating flange.

The canister is slid into and secured to the base that provides the means to vibrate the vessel at approximately 1000 Hz with an amplitude of approximately 5 mm. The vibration fluidises the iron particle bed, because the steam flow is insufficient to do this unaided. Fluidisation not only enhance the passage of the reaction steam through the bed, but helps to reduce short-circuiting, reduces the potential for particle agglomeration and typically assists in the removal of the reaction inhibiting oxide layer from partially reacted particles.

The fuel canister is secured to the retainer using a clamp that is hinged on one side and can be tightened at the accessible side over a retaining ring welded to the base of the canister. The vibration exciter is mounted underneath the canister container baseplate. The vibration exciter has a centre of mass in line with the canister vertical centre line. A variable speed hazardous-area-rated motor rotates an exciter arm at a nominal speed of approximately 1000 rpm. The exciter arm includes 5 kg eccentric mass with an adjustable shaft offset to provide an equivalent of a 50 mm load offset. A split plumber block bearing supports the non-drive end of the rotating mass.

Three equally spaced runners on the vessel are located between peripherally mounted roller bearings in the frame to restrict any significant movement away from the vertical axis. The rollers are located to either side of the frame and are capable of movement towards and away from the canister, so that the canister can be slid into place during installation after the rollers are pivoted away from the canister.

Stop means are integrated into the base plate in order to limit the maximum vibration amplitude to 10 mm so as to avoid excessive stress of being placed on the inlet-outlet connection pipe connected to the top of the vessel.

During startup, the fuel canister/reactor is purged with dry nitrogen. This ensures that any air and moisture are displaced from the canister. As the steam supply heats up, and pressurizes the steam system, a mixture of nitrogen and steam enters their reactor via the steam inlet ports on the filter vessel. This mixing of dry nitrogen and steam is continued until the reaction temperature exceeds the saturation temperature of the reaction steam. Apart from eliminating explosion risks, this action reduces the risk of iron hydroxides forming and the presence of liquid water, as these may cause particle agglomeration and loss of reactive surface area.

The steam sourced from the steam generator during startup is heated by two electric elements. One is immersed in the bottom of the vapour condenser/steam generator and generates saturated vapour, and the other is located in the top of the vapour condenser/steam generator to superheat the vapour as condensation is undesirable downstream of the steam generator. The steam is recycled between the condenser and the steam generator until the steam is superheated in excess of 50°C. Electric heating continues until a minimum reaction steam temperature is achieved, namely approximately 45O⁰C.

To achieve shutdown, all external heating is shut down and the reaction steam is quenched to approximately 250°C using an attemperation control system, until the fuel canister temperature falls below approximately 320°C. At this point, a steam bypass valve can be opened to divert the steam away from the reactor and all remaining cooling/purging of the reactor vessel can be completed with nitrogen.

As stated above, the other important vessel in the system is the vapour condenser/steam generator.

The pipe-in-pipe transfer pipe described above is typically divided using a manifold and the steam/hydrogen mixture enters the condenser via flexible braided hoses to a manifold and then into the particular top flange on each of the vapour condenser vessels of which there will typically be a plurality.

The preferred design includes three seam welded tubular vessels. Each vessel has a large height to diameter ratio. Located inside each vessel is a concentric heat transfer coil, with one coil located inside the second. Total coil length is calculated according to an average in transfer rate. For example for an average of 25 kW heat transfer rate, approximately 79 m of coil is required.

The shell of each vessel is partly filled with deionised water, ideally < 1 mg per litre of dissolved solids, < 1 mg per litre suspended solids, and >one megaohm conductivity (< 1 microsiemens conductivity). Normally, a high proportion of the deionised water required to complete a batch is initially present in the bottom of each vessel, with the remainder sourced from a condensate holding tank provided in the system.

Heat is transferred initially to the steam in the upper part of the vapour condenser/steam generator shell and then into the boiling water located in the lower part of the vessel. The steam/hydrogen coils exit via the bottom flange. Water entry and steam exit ports are located through the lower and upper shell walls. There is also an entry point in the lower third of the vessel where hot condensate recovered from the steam/hydrogen stream is returned.

The cooling of the steam/hydrogen mixture causes condensation of the steam vapour until the saturation temperature at that operating pressure is reached. For a 1000 kPag operating pressure, this is approximately 184°C. The condensate is returned to the steam generator shell by the condensate return pump. Retention of condensate in the holding tank is used as a secondary means to control the reactor temperature, that is, retaining condensate will result in increased steam flow as the condenser water level falls and the temperature rises.

The primary means for temperature control in the reaction is through the removal of condensate to the reaction steam, that is steam attemperation by progressively diverting the condensate to a spray nozzle installed in the vapour condenser/steam generator. The final steam temperature exiting the vapour condenser/steam generator is monitored to ensure it does not fall below 250°C in order to avoid condensate droplet formation. It is important that the system is operated so that it does not result in large thermal shocks, hence the maximum condensate flow is limited through appropriate pipe size selection and the configuration and operation of the automatic control system.

Electric heating elements in both the upper and lower sections of the first steam generator vessel are used for start-up. The lower elements generates saturated steam that the other element then superheats.

The use of an electric heating system for start-up allows more precise control of heating rates and peak component temperatures. To some degree this is inherently managed through the appropriate sizing of the heating elements. For example, using a 3,200 W lower element and a 1200 W upper element limits the maximum thermal ramp up rate to 4.4 kW.

During startup, and to a lesser extent during shutdown, it is important not to allow condensation to occur inside the reactor or downstream of the reactor, as liquid water may not pass through the filter elements at the necessary rate and entrained water droplets traveling at high velocity may also cause damage to the components. The presence of liquid water may also cause the development of hydroxides or corrosion products that may adversely affect both the vessel and the filter elements. Hence, the steam is initially recirculated through the condenser. During this time, the reactor is also electrically heated. Once the two vessels have been heated to approximately 220°C, steam is allowed to pass to the reactor to continue the heating sequence.

As the system ramps up, the combination of increased condenser temperature and hydrogen formation reactions will result in the system pressure rising until the operating pressure of 1000 kPa is achieved.

In the present specification and claims (if any), the word "comprising" and its derivatives including "comprises" and "comprise" include each of the stated integers but does not exclude the inclusion of one or more further integers.

Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more combinations.

Claims

Claim

1. A batch system for the production of hydrogen by thermal decomposition of water, the system including a fuel container adapted for use as a reactor, the fuel including an iron-based fuel source, a steam inlet to the fuel container to provide steam at an elevated temperature and pressure and a product gas outlet from the fuel container.

2. A batch system for the production of hydrogen as claimed in claim 1 wherein the fundamental reactions assumed in the reactor include:

Fe + 4/3.H₂O => 1/3 Fe₃O₄ + 4/3.H₂ Heat Released: 56.6kJ/mol (1013kJ/kg Fe) Fe + 2.H₂O(D => ^{1/2 Fe}2θa + 2.H_{2 (g)} Heat Released: 58.3kJ/mol (685kJ/kg Fe) Fe + H₂O ₍i) => FeO + H_{2 (g)} Heat Required: 18.8kJ/mol (337kJ/kg Fe) wherein the heat of each reaction described about is at a reference temperature of25°C.

3. A batch system for the production of hydrogen as claimed in claim 1 or claim 2 wherein magnetite (Fe₃O₄), hematite (Fe₂O₃) oxide forms, and other forms such as iron hydroxides, Fe(OH)₃. xH₂0 are formed in the reactor.

4. A batch system for the production of hydrogen as claimed in any one of the preceding claims wherein the reactor is a pre-filled fuel canister installed into the system to function as the reactor vessel until the iron-based fuel source contained in the pre-filled canister is sufficiently exhausted.

5. A batch system for the production of hydrogen as claimed in claim 4 wherein the fuel canister is also used for in situ regeneration of the spent fuel.

6. A batch system for the production of hydrogen as claimed in claim 4 wherein the fuel canister is removed from the system when the iron-based fuel material contained in the pre-filled canister is sufficiently exhausted.

7. A batch system for the production of hydrogen as claimed in any one of the preceding claims wherein the iron-based fuel source is provided as an iron particulate material.

8. A batch system for the production of hydrogen as claimed in any one of the preceding claims wherein the iron-based fuel source is provided as a high surface area porous iron particulate material.

9. A batch system for the production of hydrogen as claimed in either claim 8 or claim 9 wherein to provide improved conditions for steam to pass evenly through the particulate material, the particulate material in the reactor vessel is fluidized.

10. A batch system for the production of hydrogen as claimed in claim 8 wherein the fluidisation is acheived by mechanical vibration.

11. A batch system for the production of hydrogen as claimed in any one of the preceding claims wherein heat is liberated in the reactor so that any product gas is at a higher temperature than the reaction steam with t least a portion of the heat of the product gas is used to pre-heat the inlet steam

12. A batch system for the production of hydrogen as claimed in any one of claims 1 to 10 wherein heat is liberated in the reactor so that any product gas is at a higher temperature than the reaction steam with at least a portion of the heat of the product gas is retained in-system to maintain the temperature in the reactor.

13. A batch system for the production of hydrogen as claimed in any one of the preceding claims wherein any product gas is at least partially cooled by exchange with inlet steam via a heat exchanger.

14. A batch system for the production of hydrogen as claimed in claim 13 wherein vapour condensation of any product gas and steam generation of inlet steam are integrated as a single unit operation.

15. A batch system for the production of hydrogen as claimed in any one of the preceding claims wherein any product gas undergoes a separation process to remove water from hydrogen product gas and the hydrogen product gas is stored at a final system pressure not be greater than 1000 kPag.

16. A batch system for the production of hydrogen as claimed in any one of the preceding claims wherein the reactor is associated with a temperature regulation system to control the temperature in the reactor.